1. Field of the Invention
The present invention generally relates to gas compressor systems and, more particularly, to an electric switch gauge control apparatus for a rotary screw compressor.
2. Description of Related Art
Helical lobe rotary compressors, or "screw compressors," are well-known in the refrigeration and natural gas processing industries. This type of gas compressor generally includes two cylindrical rotors mounted on separate shafts inside a hollow, double-barreled casing. The side walls of such compressor casings typically form two parallel, overlapping cylinders which house the rotors side-by-side, with their shafts parallel to the ground.
As the name implies, screw compressor rotors have helically extending lobes and grooves on their outer surfaces. During operation, the lobes on one rotor mesh with the corresponding grooves on the other rotor to form a series of chevron-shaped gaps between the rotors. These gaps form a continuous compression chamber that communicates with the compressor inlet opening, or "port," at one end of the casing and continuously reduces in volume as the rotors turn and compress the gas toward a discharge port at the opposite end of the casing. The compressor inlet is sometimes also referred to as the "suction" or "low pressure side" while the discharge is referred to as the "outlet" or "high pressure side."
Compressor operations are sometimes described in terms of a "pressure ratio" comparing the discharge pressure produced at the compressor outlet to the pressure of the gas supplied at the compressor inlet. Since the pressure and volume of a gaseous fluid are related by the its ratio of specific heats, pressure ratios are sometimes alternatively expressed in terms of a "volume ratio." Compressor operations are also described in terms of the volumetric flow rate of the gas flowing through the compressor referred to as "capacity." However, this latter term must be considered in the context in which it is used since it can also refer to the maximum output of a particular device.
U.S. Pat. No. 4,080,110 discloses a control system for a variable capacity compressor which senses changes in electrical current flow to a motor. A current transformer and a converter provide a first electrical signal which is proportional to compressor capacity for comparison against a second signal which proportional to a system condition. The first and second signals are compared by a proportioning relay to provide a third signal for adjusting a slide valve so as to regulate compressor capacity.
The rate at which energy is consumed by the compression process is generally referred to as the "load" on the compressor and is typically expressed in units of "horsepower." The load on a compressor is mainly a function of the volume ratio and capacity of the compressor. In broad terms, the load on a compressor changes in proportion to the product of the volume ratio and capacity at which the compressor is operated. Consequently, compressor horsepower increases when either the volume ratio or capacity of the compressor are increased. Similarly, compressor load decreases when the volume ratio or capacity are decreased.
The maximum anticipated compressor load is a significant factor for determining the power output required from the prime mover which rotates the compressor. An underpowered prime mover can be easily overloaded and damaged by the compressor. An overpowered prime mover unnecessarily increases the initial cost of the compression system and may overspeed the compressor. It is therefore important to match the power output of the prime mover to the anticipated load on the compressor and/or to control the load on the compressor so as not overload the available power output from the compressor.
It is particularly important to control the load on the compressor during start-up and shut-down when the compressor is likely to receive its highest and lowest loads, respectively. For example, most screw compressor manufacturers recommend gradually loading screw compressors during start-up in order to prevent overpowering the prime mover. A typical start-up procedure begins with blocking the inlet gas source and opening the suction and discharge lines to atmosphere (to minimize volume ratio). Then, the compressor is slowly rotated (to increase capacity) before bypassing the compressor discharge line to the suction line (to further increase capacity). Finally, the gas supply to the compressor inlet is opened while slowly opening the compressor discharge to the back pressure of the process (to increase volume ratio). Similar procedures are used to gradually unload the compressor during shut-down to prevent the prime mover from over-speeding. A variety of other start-up and shut-down procedures are also well known.
In addition to matching the compressor load to the power output of the prime mover, the volume ratio and capacity of the compressor must be individually matched to the requirements of the downstream process. For example, if the volume ratio of the compressor is too high, the compressor may discharge compressed gas at a higher pressure than is required by the downstream process. Alternatively, if the capacity of the compressor is too high, the compressor will draw down the pressure of the low pressure gas source. Both of these energy inefficient operating conditions cause an unnecessary increase to the load on the compressor and thus waste a portion of the power being supplied by the prime mover.
It is well known that the volume ratio and capacity of a screw compressors can be adjusted using a slide stop and slide valve arrangement. For example, U.S. Pat. No. 4,678,406 discloses a typical configuration where the compressor operates at full capacity when the slide valve and slide stop are in contact with each other as shown in FIGS. 1-3 of the patent. In that patent, the position of the two slides together controls the volume (and pressure) ratio of the compressor and their position is adjusted in response to a signal from a pressure sensor connected to the discharge line from the compressor. When the discharge pressure drops below a set value, the slides move toward the discharge end of the compressor in order to increase the volume ratio of the compressor and prevent under compression of the gas. When the discharge pressures rises above a different value, the slides move in the other direction to decrease the volume ratio and prevent over-compression of the gas. FIG. 4 of U.S. Pat. No. 4,678,406 illustrates a slightly different configuration for operating the compressor at less than full capacity where the slide valve and stop are separated by a gap. In FIG. 4 of that patent, the position of the slide stop, and hence capacity, are adjusted in response to a signal from a pressure sensor in the suction line.
As noted above, the load on a compressor is mainly a function of the volume ratio times the capacity of the compressor. Although the arrangement discussed with respect to above can change the compressor load by individually adjusting the volume ratio and/or the capacity, it cannot sense the actual compressor load because there is no way to multiply these two variables. Thus, that arrangement cannot be used to control the load on the compressor.
One solution to this problem is to provide some type of additional control means for calculating compressor load based upon volume ratio, capacity, and/or other process variables. For example, U.S. Pat. No. 4,336,001 discloses a solid state compressor control system which indirectly calculates compressor load based on the position of a slide valve. Since the position of the slide valve indicates mostly the volume ratio of the compressor, the control system must make an assumption about capacity in order to calculate load. The safest assumption to use is that the compressor is operating at maximum capacity so that the actual load on the compressor is always maintained at less than the calculated load.
Although this assumption results in a margin of safety when the compressor is operated at less than full capacity, it also limits the volume ratio at which the compressor will operate at less than full capacity. Consequently, it does not allow the compressor load to be matched to the load capacity of the prime mover. In addition, such computer control systems are often complex, and therefore, difficult and expensive to implement, operate, and maintain.